Dimensionally stable, wipe-on, semi-crystalline-polyester-based adhesive compound

ABSTRACT

The present invention relates to a dimensionally stable adhesive compound which is formulated so as to be largely water- and solvent-free and can be brought into contact with the substrate to be bonded by being wiped on in the form of a thin adhesive film that is suitable in particular for bonding materials obtained from plant fibers. The organic constituent of the adhesive compound according to the invention consists predominantly of polyester polyurethanes, at least some of which are based on semi-crystalline polyester polyols. The invention also relates to a method for applying a tacky film to a planar substrate, preferably to paper, by wiping the adhesive compound onto the point of contact with the substrate.

The present invention relates to a dimensionally stable adhesive compound which is formulated so as to be largely water- and solvent-free and can be brought into contact with the substrate to be bonded by being wiped on in the form of a thin adhesive film that is suitable in particular for bonding materials obtained from plant fibers. The organic constituent of the adhesive compound according to the invention consists predominantly of polyester polyurethanes, at least some of which are based on semi-crystalline polyester polyols. The invention also relates to a method for applying a tacky film to a planar substrate, preferably to paper, by wiping the adhesive compound onto the point of contact with the substrate.

Dimensionally stable adhesive compounds, which are used in such a way that a thin film of the adhesive compound is transferred to the substrate by being wiped onto the point of contact, are known in the prior art and have long been implemented, for example in glue sticks for bonding paper. WO 99/51699 A1 describes glue sticks that can be smoothly wiped on but are dimensionally stable, consisting of an aqueous preparation of starch ethers and sucrose as the adhesive component and a soap gel as the shaping builder substance and, if desired, further auxiliaries. After a thin film has been wiped on, such adhesive compounds set solely through the loss of water and thus in a purely physical manner. In order to ensure the applicability of such an adhesive compound over a longer period of time, it is necessary to prevent the adhesive compound from drying out, which is made possible by the selection of a suitable, largely water-vapor-impermeable packaging means. The packaging means is often at the same time an applicator which, as a rigid structure into which the adhesive compound is filled, is also dimensionally stabilizing, so that there is also greater latitude in the formulation of the dimensionally stable adhesive compound. DE 10047069 A1 describes such an applicator for a water-based, dimensionally stable glue stick.

In contrast, water-free, dimensionally stable adhesive compounds are also known which are likewise formulated to be able to be wiped on. WO 96/37566 A1 describes semi-crystalline, dimensionally stable adhesive compounds based on polyesters and/or polyurethanes of which the semi-crystalline structure is broken up by the heat released with friction on a substrate surface, such that a thin adhesive film can be applied which, after a short time, forms semi-crystalline regions again and in this way sets and is able to bond the substrate. In said document, various additives which modify the crystallinity or increase tackiness or which are pigments, fillers, plasticizers, dyes, antioxidants and preservatives are added to such adhesive compounds to improve the property profile. In principle, the adhesive compounds disclosed in the WO publication are already suitable for being placed on the market largely without packaging and for being applied manually to substrates, in particular paper, without a special applicator.

Such a largely water- and solvent-free, dimensionally stable adhesive compound must, however, satisfy a complex requirement profile that includes application-specific, consumer-specific and environmental hygiene aspects. For this purpose, the property profile of dimensionally stable adhesive compounds which can be activated by friction must be further improved in order to be able to establish packaging-free sales units in the consumer market. In this context, it is important to find formulations that can be easily wiped on in the form of a tacky film on flat substrates, have sufficient initial tackiness that flat substrates can be fixed in their relative position with respect to one another via the adhesive film, and set in a short time in a precisely positioned manner and in the process form a cohesive bond that is as long-lasting as possible. Ideally, the adhesive compound is suitable for bonding materials obtained from plant fibers. This property profile is also intended to be retained even after the adhesive compound is stored in a way that is customary in trade and typical for use.

On the part of the consumer, it is also expected that a dimensionally stable adhesive compound which can be wiped onto the substrate directly by hand without an applicator has no tackiness, that the skin is not soiled with adhesive compound and that skin compatibility is ensured. It is also expected by the consumer that such an adhesive compound, in the form of a stick, can be guided over the substrate like a wax crayon when in use, and a continuous adhesive film can be applied in the process. Wiping the adhesive compound on a flat substrate must be possible with little effort, so that flexible thin substrates such as paper are not warped or torn when the adhesive is applied.

Overall, the present invention thus addresses the problem of providing a dimensionally stable, wipe-on adhesive compound which can be formulated so as to be water- and solvent-free and skin-friendly and yet can be easily applied without a separate applicator and for this purpose in particular has a low wiping resistance, with it being intended that the adhesive compound is not permanently tacky at 30° C. Temperature stability of up to 70° C. is also required for storage and transport. Above all, the adhesive compound is intended to have a high adhesive strength for materials obtained from plant fibers, in particular paper and cardboard. In addition, an adhesive film wiped on by the adhesive compound is intended to generate sufficient initial adhesion that the user does not have to press the substrates to be bonded until they set in order to achieve adequate bonding.

This range of problems is surprisingly solved by formulating an adhesive compound of which the organic constituent comprises polyester polyurethanes, at least some of which are based on semi-crystalline polyester polyols.

The present invention accordingly relates to a dimensionally stable adhesive compound of which the organic constituent consists predominantly of polyester polyurethanes, at least some of which are based on semi-crystalline polyester polyols.

Semi-crystalline polyester polyols within the meaning of the present invention are those which can be obtained by polycondensation of a reaction mixture comprising one or more dicarboxylic acids selected from terminal dicarboxylic acids that are both saturated linear and aliphatic and have an even number of at least 8 methylene groups (hereinafter “component A”), and one or more diols selected from terminal diols that are both saturated linear and aliphatic and contain at least one ether function (hereinafter “component B”).

In connection with semi-crystalline polyester polyols, a dicarboxylic acid or a diol is considered to be aliphatic if, aside from oxygen atoms, it is composed exclusively of carbon and hydrogen atoms, the term “dicarboxylic acid” also including the corresponding single or double methyl and/or ethyl esters as well as the corresponding single or double acid chlorides. In this context, it also applies that, in the case of a terminal dicarboxylic acid or a terminal diol, the two carboxyl and hydroxyl groups, respectively, are at the end.

An aliphatic compound is linear if it contains neither tertiary nor quaternary carbon atoms, with one carbon atom being quaternary if it is covalently bonded to four other carbon atoms, while a tertiary carbon atom is covalently bonded to three other carbon atoms.

The polyester polyurethanes necessarily contained according to the invention based on semi-crystalline polyester polypols have crystalline phases below their ring-and-ball softening point, measured in accordance with DIN EN 1238:2011, which phases are detected by differential scanning calorimetry (DSC) at a heating rate of not more than 10 Kelvin per minute as an endothermic melting peak before the softening point is reached.

In a particular embodiment, polyester polyurethanes are incorporated which are based on semi-crystalline polyester polyols that recrystallize on melting and that can be obtained by polycondensation of a reaction mixture comprising one or more dicarboxylic acids and one or more diols,

-   a) at least 50 mol. %, preferably at least 60 mol. %, particularly     preferably at least 70 mol. %, of the dicarboxylic acids being     selected from terminal dicarboxylic acids having an even number of     at least 8 methylene groups that are both saturated linear and     aliphatic (hereinafter “component A”); and -   b) at least 40 mol. %, preferably at least 60 mol. %, particularly     preferably at least 70 mol. %, of the diols being selected from     terminal diols that are both saturated linear and aliphatic and have     at least one ether function (hereinafter “component B”).

Such semi-crystalline polyester polyols which recrystallize during melting are distinguished by the fact that exothermic recrystallization occurs during the endothermic melting process before the softening point is reached, i.e., before the transition to the liquid phase, so that an exothermic crystallization peak superimposed on the endothermic melting peak can also be detected by differential scanning calorimetry (DSC) at a heating rate of not more than 10 Kelvin per minute before the softening point is reached. Corresponding semi-crystalline polyester polyols that recrystallize on melting are described in detail in WO 2019/011529 A1. In the context of the present invention, the polyester polyurethanes produced on the basis of these semi-crystalline polyester polyols which recrystallize on melting, as a constituent of the adhesive compound according to the invention, impart excellent wiping behavior together with high temperature stability. The application of an adhesive thin film to the substrate to be bonded is thus facilitated without the softening point of the adhesive compound being significantly lowered and the packaging-free, dimensionally stable adhesive being perceived by the end consumer as being tacky to use.

In order not to use semi-crystalline polyester polyols which give the polyester polyurethanes a softening point above 60° C. and therefore require addition of additives to enhance the wiping of the adhesive, it is preferred that the dicarboxylic acids according to component A do not have more than 24 methylene groups, particularly preferably not more than 18 methylene groups, more particularly preferably not more than 16 methylene groups, with, independently of this, the dicarboxylic acids according to component A preferably needing to comprise at least 10 methylene groups in order to provide those semi-crystalline polyester polyols which, due to their pronounced tendency to recrystallize, impart excellent wiping properties as the basis for the polyester polyurethanes of the adhesive compound. The preferred representatives of dicarboxylic acids according to component A are therefore 1,10-decamethylenedicarboxylic acid, 1,12-dodecamethylenedicarboxylic acid, 1,14-tetradecamethylenedicarboxylic acid and 1,16-hexadecamethylenedicarboxylic acid.

To improve the tackiness of the adhesive compound, it can be advantageous if those polyester polyurethanes are contained which are based on polyester polyols which are obtained from the polycondensation of aromatic dicarboxylic acids or unsaturated dicarboxylic acids having fewer than 9 carbon atoms with aliphatic diols, which preferably have at least one ether function, but have neither a tertiary nor a quaternary carbon atom. Isophthalic acid, terephthalic acid, orthophthalic acid, and furandicarboxylic acid, in particular 2,5-furandicarboxylic acid, can be mentioned in particular as aromatic dicarboxylic acids, and itaconic acid, fumaric acid and/or maleic acid, which can also be contained in the adhesive compound as a polyester polyol constituent of the polyester polyurethanes, can be mentioned as unsaturated dicarboxylic acids having fewer than 9 carbon atoms.

Overall, however, it is advantageous for the wiping behavior and the temperature stability of the adhesive compound according to the invention if the proportion of polyester polyurethanes that is based on semi-crystalline polyester polyols is predominant based on the entirety of the polyester polyurethanes. According to the present invention, this is the case when the proportion of the dicarboxylic acids that can be formally separated from the polyester polyurethanes by hydrolysis of the ester functions according to component A and the diols that can be separated according to component B, based on the polyester polyol proportion of all the polyester polyurethanes contained in the adhesive compound according to the invention, is at least 60 wt. %, preferably at least 80 wt. % and particularly preferably at least 90 wt. %.

Semi-crystalline polyester polyols which have an acid number preferably less than 50 mg KOH/g, particularly preferably less than 10 mg KOH/g, more particularly preferably less than 5 mg KOH/g and very particularly preferably less than 2 mg KOH/g, in each case per gram of the reaction mixture comprising components A and B and which can be obtained from a reaction mixture of components A and B, the diols being contained in molar excess based on the dicarboxylic acids, but not above a molar excess of 1.2:1, have, as starting materials for the polyester polyurethanes described above, proven to be ideal in terms of recrystallization behavior of the adhesive compound according to the invention.

The acid number is, according to the invention, an experimentally determinable measured variable which is a measure for the number of free acid groups in the relevant defined reference amount, for example per gram of the reaction mixture. The acid number is determined by dissolving a weighed sample of the reference amount in a solvent mixture of methanol and distilled water in a volume ratio of 3:1, and then potentiometrically titrating said sample with 0.05 mol/1 KOH in methanol. The potentiometric measuring is carried out using a combination electrode (LL-Solvotrode® from Methrom; reference electrolyte: 0.4 mol/1 tetraethylammonium bromide in ethylene glycol). The acid number in this case corresponds to the added amount of KOH per gram of the reference amount at the inflection point of the potentiometric titration curve.

For the provision of the polyester polyurethanes via the addition of mono- or polyfunctional isocyanates to such semi-crystalline polyester polyols, it is preferred if the hydroxyl number of the polyester polyols is at least 30 mg/g KOH, particularly preferably at least 60 mg/g KOH, more particularly preferably at least 80 mg/g KOH, but preferably has a hydroxyl number below 200 mg/g KOH, particularly preferably below 140 mg/g KOH, more particularly preferably below 120 mg/g KOH, in each case per gram of the reaction mixture comprising components A and B.

Similarly to the acid number, the hydroxyl number can be experimentally determined by means of potentiometric titration, as a measure for the number of free hydroxyl groups in the relevant defined reference amount, for example per gram of the reaction mixture. For this purpose, a weighed sample of the reference amount is heated in a reaction solution of 0.1 mol/l phthalic anhydride in pyridine at 130° C. for 45 minutes, and is mixed with 1.5 times the volume of the reaction solution of pyridine, and is then mixed with 1.5 times the volume of the reaction solution of deionized water (κ<1 μScm⁻¹). The released amount of phthalic acid is titrated in this mixture by means of 1 M sodium hydroxide solution. The potentiometric measuring is carried out using a combination electrode (LL-Solvotrode®from Methrom; reference electrolyte: 0.4 mold tetraethylammonium bromide in ethylene glycol). The hydroxyl number in this case corresponds to the added amount of NaOH per gram of the reference amount at the inflection point of the potentiometric titration curve.

The semi-crystalline polyester polyols which can be obtained from the reaction mixture containing the components A and B defined above preferably have a number-average molar mass of less than 5,000 g/mol, particularly preferably less than 2,000 g/mol, but more preferably at least 500 g/mol, the polydispersity preferably being less than 2.5. In this molar mass range and with the preferred polydispersity, polyester polyurethanes can be obtained which allow the adhesive compounds according to the invention, which consist of at least 50 wt. % of such polyester polyurethanes, to be wiped on easily and to give said compounds good tackiness together with sufficient temperature stability. The number-average molar mass can be determined using a sample of the reaction mixture, by means of gel permeation chromatography after calibration against polystyrene standards. For this purpose, at a column oven temperature of 40° C., chromatography is carried out by elution with tetrahydrofuran, with the distribution curve being recorded by means of a concentration-dependent RI detector which continuously determines the refractive index in the eluate at a temperature of likewise 40° C.

Polyester polyurethanes of the adhesive compound according to the invention are addition products of monofunctional or polyfunctional isocyanates with polyester polyols, the addition product being end-capped and thus not having any free isocyanate groups. It is preferred according to the invention if the addition is carried out in such a way that the molar ratio of isocyanate groups to hydroxyl groups of the polyester polyol is greater than 2, preferably greater than 2.5, but preferably less than 4, particularly preferably less than 3.5. The end-capping is preferably carried out with monohydric alcohols. Within the meaning of the present invention, a polyester polyurethane does not contain any free isocyanate groups if it contains less than 0.1 wt. % of NCO, based on the total amount of polyester polyurethanes. To determine the proportion of free isocyanate groups, 4 milliliters of a 0.5 M dibutylamine solution in xylene are added to 1 gram of the polyester polyurethane and then mixed with 50 milliliters of xylene and stirred at a temperature of 20° C. until completely homogenized. After adding 3 drops of bromophenol blue and 50 milliliters of isopropanol, titration is carried out with 0.5 M hydrochloric acid until the color changed from blue to yellow. The difference in the consumption of hydrochloric acid for titration of a blank value solution, i.e., without the addition of the polyester polyurethane, multiplied by the value 2.1 and divided by the exact weight of the polyester polyurethane gives the percentage by weight of isocyanate groups.

According to the invention, the polyester polyurethanes are preferably obtained by adding at least trifunctional, particularly preferably aliphatic, isocyanates, which are very particularly preferably selected from trimers of hexamethylene diisocyanate and/or pentamethylene diisocyanate, to polyester polyols, preferably to semi-crystalline polyester polyols, with free isocyanate groups formed in the addition reaction preferably being reacted (hereinafter “end-capping”) with at least one monohydric alcohol, particularly preferably with at least one aliphatic alcohol, more particularly preferably with at least one linear aliphatic alcohol, each of which preferably has not more than 24 carbon atoms in the main chain, but preferably has at least 4 carbon atoms in the main chain. In order to increase the tackiness, it is advantageous if the end-capping is carried out with monohydric aliphatic alcohols which have at least 4 carbon atoms but fewer than 8 carbon atoms in the main chain, whereas an increased temperature stability of the adhesive compound is achieved if the end-capping is carried out with monohydric, preferably linear aliphatic alcohols which have at least 12 carbon atoms but fewer than 24 carbon atoms in the main chain. Via the relative ratio of long-chain to short-chain end-capped polyester polyurethanes, the tackiness and temperature stability required for the specific application can accordingly be set, in particular also depending on and in the presence of specific further additives. In a preferred embodiment of the adhesive compound according to the invention, the polyester polyurethanes are end-capped both with monohydric aliphatic, preferably linear aliphatic alcohols having at least 4 but fewer than 10, preferably not more than 8, carbon atoms, and with monohydric aliphatic, preferably linear aliphatic alcohols having at least 10, preferably at least 12, particularly preferably at least 14, but not more than 24 carbon atoms.

Polyester polyurethanes having a dynamic Brookfield viscosity (spindle 27) at 80° C. of at least 2,000 mPas, particularly preferably at least 3,000 mPas, more particularly preferably at least 5,000 mPas, but preferably less than 140,000 mPas, particularly preferably less than 60,000 mPas, are also preferably used. Polyester polyurethanes having a dynamic viscosity in the lower preferred range are usually achieved by the end-capping being carried out at least in part with long-chain monohydric alcohols, or by an addition of divalent isocyanates to the semi-crystalline polyester polyols being carried out at least in part.

The organic constituent of the adhesive compound according to the invention then consists predominantly of polyester polyurethanes if their proportion is greater than 50 wt. %, based on the organic constituent of the adhesive compound. The adhesive compounds usually exhibit particularly good temperature stability together with good wiping behavior when the proportion of polyester polyurethanes, as described above, based on the organic constituent of the adhesive compound, is at least 60 wt. %, preferably at least 70 wt. % or even more preferably at least 80 wt. %, so that such adhesive compounds are preferred according to the invention.

Due to the excellent wiping behavior, together with temperature stability and good setting behavior, imparted to the adhesive compound according to the invention via the polyester polyurethanes, extensive addition of additives can be largely dispensed with. Nevertheless, the temperature stability can be increased further without having to accept significant losses in terms of the initial tackiness of the wiped-on film. For this purpose, polyester polyurethanes which can be obtained preferably by adding at least trifunctional isocyanates to polyester polyols, particularly preferably to polytetramethylene ether glycol, and then end-capping with at least one monohydric alcohol, preferably with at least one aliphatic alcohol, particularly preferably with at least one linear aliphatic monohydric alcohol, each of which preferably has not more than 24 carbon atoms in the main chain, but preferably has at least 4 carbon atoms in the main chain, can be treated with additives. In a particular embodiment, the at least trifunctional isocyanate can be added to a mixture of the above-described semi-crystalline polyester polyols, which mixture additionally also contains the polyether polyols. As a result, mixed polyester-polyether polyurethanes are obtained which also meet the requirement profile for an adhesive compound according to the invention and impart good wipeability together with excellent temperature stability, provided that the proportion of polyester polyols is predominant and is more than 50 wt. % based on the polyol proportion of the reaction mixture of the addition reaction.

The proportion of polyether polyurethanes based on the organic constituent of the adhesive compound in a preferred adhesive compound according to the invention is not more than 40 wt. %, particularly preferably not more than 30 wt. %, more particularly preferably not more than 20 wt. %, very particularly preferably not more than 15 wt. %, but preferably at least 2 wt. %, particularly preferably at least 5 wt. %, in order to be able to obtain colorlessly translucent adhesive compounds.

Listed below are further additives and their relevant functionality which can advantageously be added to the adhesive compound according to the invention in order to provide adhesive compounds which meet the specific requirement profiles, as set out further below. One advantage of the adhesive compounds according to the invention is accordingly that it is easy to add additives to the adhesive compound, which consists predominantly of the polyester polyurethanes.

To increase the initial tackiness of a wipe-on film of the adhesive compound, it can be advantageous, for example, based on the adhesive compound according to the invention, for up to 5 wt. % of tackifiers that are not polyester polyurethanes or polyether polyurethanes to be additionally contained. For a significant increase in the initial adhesion, a proportion of at least 1 wt. % of tackifiers, based on the adhesive compound, is preferred. Such tackifiers are organic compounds which preferably have a sum of hydroxyl and acid numbers above 100 mg KOH per gram of the relevant organic compound. The tackifiers are preferably resins, preferably, due to their sustainable availability and largely harmless nature with respect to environmental hygiene, natural resins which can be chemically modified. According to the invention, a natural resin according to DIN 55958 comprises excretions secreted by animals and plants. Resins of this type are known to a person skilled in the art, for example as turpentine, balsam, gum lacquer, rosin, sandarac or mastic. According to the invention, the natural resins also include the modified natural resins which are obtained from the natural resins, for example by hydrogenation, intrinsic addition reactions or esterification. The natural resins as component a) of the adhesive compound according to the invention are in turn preferably selected from resin acids and/or resin esters, particularly preferably from resin acids and/or resin esters based on di- and/or triterpenes, which are preferably hydrogenated and/or are present as intrinsic addition products. According to the invention, the term resin ester describes the resin acids which have been modified once or several times via the carboxyl groups of the resin acids in condensation reactions with the formation of ester groups. Preferred representatives of the diterpene-based resin acids and/or resin esters are accordingly abietic acid, neoabietic acid, levopimaric acid, pimaric acid, palustric acid, agathenic acid, illuric acid and podocarpic acid, and the triterpene-based resin acids are elemic acid, sumarresinoleic acid and their mono- or polyesters, having preferably in each case not more than 6 carbon atoms in the alkyl group of the ester group. The use of tackifiers gives the adhesive compound, after it has been wiped in the form of a thin film onto a paper surface, increased initial adhesion, which, for example when bonding opposite sides of a paper fold, ensures that the paper surfaces connected by the adhesive film remain connected without exerting a contact pressure until the glue has set.

Small amounts of polyolefin waxes can also have a positive effect on the initial adhesion of the adhesive compound due to their largely non-polar character. In addition, the polyolefin waxes bring about better long-term stability of the bond, so that preferably up to 20 wt. %, particularly preferably up to 10 wt. %, in each case based on the adhesive compound, of polyolefin waxes can be contained according to the invention, with a proportion of at least 1 wt. %, in particular at least 2 wt. %, in each case based on the adhesive compound, being preferred for a significant effect. In a preferred embodiment of an adhesive compound according to the invention additionally comprising polyolefin waxes, these are selected from copolymer waxes composed of propylene and ethylene and/or at least one branched or unbranched 1-alkene having 4 to 20 carbon atoms, and can additionally be modified with carboxyl groups for improved compatibility with the adhesive compound, the acid number preferably being below 20 mg KOH per gram of the polyolefin wax. It is also advantageous if the additive-treated polyolefin wax is amorphous and therefore has no crystallinities in the range from 20° C. to the softening point. In addition to the initial adhesion, the polyolefin waxes also increase the elasticity of the adhesive compound, so that, for example, glue sticks based on a correspondingly additive-treated adhesive compound according to the invention can be provided which are less susceptible to breakage, which is particularly advantageous in the case of packaging-free administration and application forms of the adhesive compound according to the invention.

For adhesive compounds according to the invention of which the tackiness is perceived as too unpleasant when the adhesive compound is applied without packaging and an adhesive film is wiped on by hand, the addition of pyrogenic silicic acids up to an amount of 15 wt. %, based on the adhesive compound, can be suitable, while maintaining the other parameters such as initial adhesion, wipeability and setting behavior, for reducing the tackiness of the dimensionally stable administration form, with at least 1 wt. % of the pyrogenic silicic acid preferably needing be treated with additives for a noticeable reduction in tackiness.

In order to improve the wiping of the adhesive compound onto a flat substrate, it can also be advantageous for the adhesive compound to additionally contain up to 5 wt. % of non-ionic surfactants which preferably have an HLB value in the range of from 12-18 and are particularly preferably selected from polyalkoxylated C12-C22 fatty alcohols, very particularly preferably from polyethoxylated C12-C22 fatty alcohols having preferably more than 20 EO units. For a significant improvement in the wiping, a proportion of at least 0.5 wt. % of non-ionic surfactants, based on the adhesive compound, is preferred.

The adhesive compound according to the invention is distinguished in that it can be formulated so as to be solvent-free and water-free, and the bonded connection, due to the setting of the adhesive compound, does not require any physical loss of solvent or water. In a preferred embodiment, the adhesive compound according to the invention therefore contains less than 5 wt. %, preferably less than 1 wt. %, of water, based on the adhesive compound. The proportion of water in the adhesive compound can be determined by the Karl Fischer method in the solvent xylene. Such virtually water-free adhesive-compound formulations according to the invention have the advantage that after an adhesive film has been wiped onto a thin, flat substrate of a water-absorbent material, for example paper, said material does not warp and retains its flat shape even after said adhesive compound has set. In a further preferred embodiment, the adhesive compound according to the invention contains less than 5 wt. %, particularly preferably less than 1 wt. %, based in each case on the adhesive compound, of organic compounds having a boiling point below 100° C. at 1013 mbar.

In a particularly preferred embodiment, the adhesive compound according to the invention contains:

-   a) at least 50 wt. %, preferably at least 60 wt. %, particularly     preferably at least 70 wt. %, more particularly preferably at least     80 wt. %, of polyester polyurethanes, at least some of which     originate from semi-crystalline polyester polyols, which in turn can     be obtained by polycondensation of a reaction mixture comprising one     or more dicarboxylic acids selected from terminal dicarboxylic acids     that are both saturated linear and aliphatic and have an even number     of at least 8 methylene groups, and one or more diols selected from     terminal diols that are both saturated linear and aliphatic and     contain at least one ether function. -   b) up to 50 wt. %, preferably 5 to 30 wt. %, of polyether     polyurethanes; -   c) up to 20 wt. %, preferably 1 to 10 wt. %, of polyolefin waxes; -   d) up to 5 wt. % of tackifiers; and -   e) up to 5 wt. % of polyalkoxylated C12-C22 fatty alcohols,     in each case based on the organic constituent, the proportion of the     organic constituents not exceeding at least 80 wt. % based on the     adhesive compound, and the proportion of inorganic constituents     selected from pigments, fillers and salts not exceeding 20 wt. %,     preferably 15 wt. %, based on the adhesive compound, and     less than 5 wt. %, preferably less than 1 wt. %, based on the     adhesive compound, of water being contained.

In connection with this particularly preferred embodiment, the specific configurations of components a) to e) and further additives listed above apply analogously.

According to the invention, the inorganic constituent corresponds to the solids content of the adhesive compound that remains after pyrolysis in a reaction oven with the supply of a CO₂-free oxygen stream at 900° C. without admixture of catalysts or other additives, with pyrolysis being carried out until an infrared sensor provides a signal identical to the CO₂-free carrier gas (blank value) in the outlet of the reaction oven. Correspondingly, the organic constituent of the adhesive compound is the adhesive compound minus the amount of water determined by the Karl Fischer method in the solvent xylene and minus the inorganic constituent as defined above.

In the context of the present invention, an adhesive compound is dimensionally stable in principle when a mass of the preparation (10 grams) shaped to form a cylinder and having a base of 2 cm² deforms irreversibly, when a constantly increasing force acts upon said mass perpendicularly to the base of the cylinder at 30° C. and 50% relative humidity, only above a pressure of 20 N/cm². The action of the force and the determination of the occurrence of the deformation can be monitored by means of a force measuring device, for example by means of the Texture Analyzer TA-XT HiR (Stable Micro Systems Ltd.). The concept of dimensional stability is closely linked to the required temperature stability of adhesive compounds according to the invention, specifically such that adhesive compounds with a ring-and-ball softening point measured according to DIN EN ISO 4625-1:2006-04 of preferably at least 40° C., particularly preferably of at least 60° C., but preferably of less than 150° C., particularly preferably of less than 100° C., are considered to be temperature-stable to the extent that dimensionally stable adhesive compounds can be provided at 30° C. Ideally, at least 80 wt. % of the organic constituents of the adhesive compound according to the invention have a ring-and-ball softening point, measured according to DIN EN ISO 4625-1:2006-04, of preferably at least 40° C., particularly preferably at least 60° C., in order to provide an adhesive compound that is as dimensionally stable and temperature-stable as possible.

The dimensionally stable adhesive compound provided according to the present invention has low tackiness at 30° C. and can easily be wiped onto substrates. The transfer of material brought about by wiping takes place in such a way that a thin, tacky film of the adhesive compound results on flat substrates, which film sets after a short time so that substrates can be bonded to one another with sufficient adhesion to the adhesive compound.

In a further aspect, the present invention therefore relates to a method for applying a tacky film to a planar substrate, preferably to paper, by pressing a dimensionally stable adhesive compound according to the invention onto the planar substrate and subsequently changing the relative position while maintaining a contact pressure perpendicular to the surface normal of the substrate. Suitable substrates are preferably obtained from plant fibers, are planar and flexible, in particular paper.

Practical Examples:

In the following, first the preparation of a mixture of semi-crystalline polyester polyols is outlined, and then the conversion thereof to corresponding polyester polyurethanes, which as such were cast to form cylindrical, dimensionally stable adhesive compounds according to the invention and characterized with regard to wiping behavior and bonding of paper

Preparation of a Mixture of Polyester Polyols:

The raw materials 0.369 kg isophthalic acid, 1.876 kg dodecanedioic acid and 1.421 kg diethylene glycol were placed in a 5-liter four-necked flask equipped with a nitrogen inlet, a thermostat, a paddle stirrer and a distillation arm. The reaction mixture was heated and stirred at a temperature of 220° C. in a stream of nitrogen for several hours and then the pressure in the reaction flask was gradually reduced to 30 mbar. The acid number was controlled continuously, as explained in detail in the description of the invention. As soon as the acid number dropped below a value of 3 mg KOH/g in relation to the reaction mixture, the reaction mixture was first cooled to 80° C. and cooled, after which the reaction mixture was allowed to cool further to room temperature. Afterward, the acid number and hydroxyl number were finally determined at 20° C. and the mixture was chromatographically characterized. The hydroxyl number was 102 mg KOH/g; the acid number was 0.3 mg KOH/g and the dynamic viscosity at 50° C. was 1,900 mPas.

Preparation of a Mixture a of Polyester Polyurethanes:

51.8 g of n-octanol and 170 g of isocyanate trimer Desmodur® N3300 (Covestro AG) were placed in a three-necked flask and heated to 80° C. and stirred for 30 minutes, and then 165 g of the polyester polyol mixture and 48.7 g hexadecanol were added and the reaction mixture was stirred initially at 100° C. for one hour, then at 110° C. for about 1.5 hours and finally for a further 10 minutes at reduced pressure of 300 mbar.

For the purpose of chromatographic characterization by gel permeation chromatography (GPC), a sample of the reaction mixture was dissolved with tetrahydrofuran and applied to the column, and subsequently also eluted with tetrahydrofuran. Gel permeation chromatography (GPC) with an RI detector after calibration by means of polystyrene standards was performed at a column oven temperature of 40° C. and a temperature in the detector of likewise 40° C. The relative number-average and weight-average molar mass values were determined from the molar mass distribution curve and the polydispersity was determined therefrom.

Mixture A of the polyester polyurethanes had a number-average molar mass of approx. 3,700 g/mol with a polydispersity of 3.6.

Preparation of a Mixture B of Polyester Polyurethanes:

51.8 g of n-octanol, 170 g of isocyanate trimer Desmodur® N3300 and 165 g of the polyester polyol mixture were placed in a three-necked flask and heated to 100° C. and stirred for 30 minutes, and 48.7 g of hexadecanol were then added and the reaction mixture was stirred at 110° C. for two hours. Mixture B of the polyester polyurethanes had a number-average molar mass of approx. 3700 g/mol with a polydispersity of 8.4.

Mixtures A and B of the polyester polyurethanes were melted at 110° C. and cast in a cylindrical stick shape (diameter 2 cm, length 6-8 cm) and stored for 24 hours at 20° C. before the handling behavior of the stick-shaped adhesive compounds and the property of the wiped-on adhesive film were determined.

The wiping, determined after 10° incision of the cylindrical stick and wiping via the cut surface on paper with a grammage of 80 g/m2 and a contact pressure of 5 N/cm2, was given the value “4” for mixture A and the value “3” for mixture B (scale from 1: stiff, wiping comparable with an eraser, to 5: like Pritt® Original Stick from Henkel AG & Co. KGaA)

The paper tear, determined after bonding a sheet of paper with a grammage of 80 g/m2, which was provided with an adhesive film according to the wiping test and then folded for bonding and pulled apart again after 1 hour at 20° C., was given the value “5” (1: no paper tear, 2: less than 30%, 3: less than 60%, 4: less than 90%, 5: more than 90% of the bond area shows paper tears) for both mixtures.

The softening point of the adhesive compound or of the mixtures A and B of polyester polyurethanes, measured using the ring-and-ball method in accordance with DIN EN ISO 4625-1:2006-04, was 66° C. (mixture A) and 64° C. (mixture B). The adhesive compounds were thus sufficiently temperature-stable to be able to be offered free of packaging. 

1. A dimensionally stable adhesive compound comprising: (i) an inorganic constituent; (ii) an organic constituent comprising polyester polyurethanes based on semi-crystalline polyester polyols, wherein the semi-crystalline polyester polyols are prepared by polycondensation reaction of (A) one or more dicarboxylic acids selected from terminal dicarboxylic acids having an even number of at least 8 methylene groups that are both saturated linear and aliphatic, and (B) and one or more diols selected from terminal diols that are both saturated linear and aliphatic and contain at least one ether function; and (iii) water.
 2. The adhesive compound according to claim 1, characterized in that the polyester polyurethanes are prepared by adding at least trifunctional isocyanates to the semi-crystalline polyester polyols, and end-capping with at least one monohydric alcohol having 4-24 carbon atoms.
 3. The adhesive compound according to claim 2, characterized in that the end-capping is carried out with (i) linear aliphatic alcohols having 4 [to] 10 carbon atoms, and (ii) linear aliphatic alcohols having 10 [to] 24 carbon atoms.
 4. The adhesive compound according to claim 2, characterized in that the at least trifunctional isocyanates are aliphatic and are selected from trimers of hexamethylene diisocyanate pentamethylene diisocyanate or mixture thereof.
 5. The adhesive compound according to claim 1, characterized in that the semi-crystalline polyester polyols have a number-average molar mass of 500 to 5,000 g/mol.
 6. The adhesive compound according to claim 1, characterized in that the semi-crystalline polyester polyols have a hydroxyl number of 30 [to] 200 mg/KOH, based on the amount of the polyester polyols.
 7. The adhesive compound according to claim 1, characterized in that the polyester polyurethanes have a dynamic Brookfield viscosity (spindle 27) at 80° C. of 2,000 [to] 60,000 mPas.
 8. The adhesive compound according to claim 1, characterized in that the proportion of [the] polyester polyurethanes obtained from [the] semi-crystalline polyester polyols is at least 60 wt. %, based on the total proportion of [the] polyester polyurethanes.
 9. The adhesive compound according to claim 1, characterized in that the proportion of [the] polyester polyurethanes, based on the organic constituent of the adhesive, is at least 60 wt. %.
 10. (canceled)
 11. (canceled)
 12. (canceled)
 13. The adhesive compound according to claim 1, characterized in that [the] water is less than 1 wt % of the adhesive.
 14. The adhesive compound according to claim 1 wherein the organic constituent contains a) at least 50 wt. % of [the] polyester polyurethanes, based on the semi-crystalline polyester polyols prepared by polycondensation reaction of (i) one or more dicarboxylic acids selected from saturated linear aliphatic terminal dicarboxylic acids having an even number of at least 8 methylene groups, and (ii) one or more diols selected from linear aliphatic terminal diols that contain at least one ether function; b) 5 to 30 wt. % of polyether polyurethanes; c) 1 to 10 wt. % of polyolefin waxes; d) up to 5 wt. % of tackifiers; and e) up to 5 wt. % of polyalkoxylated C12-C22 fatty alcohols wherein the wt. % is based on the organic constituent, [and] wherein the proportion of the organic constituents does not exceed 80 wt. % based on the adhesive compound, and the proportion of inorganic constituents selected from pigments, fillers and salts does not exceed 20 wt. %, based on the adhesive compound, and water is less than 5 wt. %, based on the adhesive compound.
 15. A method for applying a tacky film to a planar paper substrate by pressing the dimensionally stable adhesive compound according to claim 1 onto the planar substrate and subsequently changing the relative position while maintaining a contact pressure perpendicularly to the surface normal of the substrate. 